Evidence clearly shows very small (<4.0cm diameter) abdominal aortic aneurysms (AAAs) do not warrant immediate surgery while larger AAAs (>6cm) pose a substantial risk of rupture and should be repaired. Other areas of AAA care, including management of small AAAs (4.0-5.5cm), optimal choice of endovascular vs open repair based on patient characteristics, and whether current use of the two repair types aligns with the evidence regarding their relative risks and benefits are less certain. Regarding management of small AAAs, two randomized controlled trials (RCTs) -the United Kingdom Small Aneurysm Trial (UKSAT) and the Aneurysm Detection and Management Trial (ADAM) in the United States- compared immediate open surgical repair to surveillance and selective surgery. Neither showed improved overall survival with either treatment, but did reveal possible survival differences within some AAA size and age subgroups. Comparisons of endovascular repair and selective surveillance in this patient population are underway but results will not be available for several years. The proposed comparative effectiveness study addresses two important areas of uncertainty in AAA management. First, a large population-based study will use AAA surgery data from an urban area (Dallas-Fort Worth) to describe current use of endovascular repair, and compare outcomes for endovascular and open repair for patient subgroups to inform clinical decisions. Aims: Aim 1: to describe elective AAA surgeries by aneurysm diameter in a large urban area over the past eight years. Aim 2: to assess changes in surgery choice (AAA open or endovascular) over the past eight years and differences in outcomes (long-term survival, re-operation and rupture) between surgery types by age, gender, AAA size, and race. Aim 3: to develop a prospective AAA surgery registry containing the demographic and clinical variables and outcomes assessed, to be available to other researchers at their request. The second set of aims address optimal management strategies for small AAAs. Neither the UKSAT nor the ADAM trial had sufficient statistical power to examine survival differences for age and AAA size subgroups but a pooled analysis of their data provides this opportunity. Aim 4: to determine, using the pooled patient-level data from the UKSAT and ADAM trials, whether there is a survival difference for immediate open repair vs. selective surveillance for patients with 4.0-5.5cm AAAs within the AAA size (4-4.4cm; 4.5-4.9cm; and 5-5.5cm) and age (d69 yr; and e70 yr) subgroups. Aim 5: to determine, from the same data, whether the survival effect of open repair vs. surveillance varies by aneurysm size within age group (or vice versa). Aim 6: to use the results regarding the effectiveness of open repair vs. surveillance to estimate the percentage of patients whose treatment decisions are affected based on the data collected in the population-based study in Aim 1. Significance: The proposed study has the potential to provide, immediately, novel data that can impact current clinical guidelines and practice for AAAs. Study results will be applicable to more than 800,000 patients the US. PUBLIC HEALTH RELEVANCE: The proposed research will provide important information for public health as it addresses both the question of the best treatment options for patients with abdominal aortic aneurysms (AAAs) between 4.0cm and 5.5cm in diameter and examines current use of endovascular AAA repair, providing an opportunity to assess whether clinical practice in this area aligns with the available evidence regarding the relative risks and benefits of open versus endovascular repair based on patients' characteristics (such as age, gender, and size of AAA). Results will be applicable to more than half million people (approximately 430,000 men and 120,000 women) in the United States and many more people worldwide. Similarly, the results of the population-based study of the 2 types of AAA repair available have the potential to impact treatment for all individuals with an AAA - approximately 614,000 men and 182,000 women in the United States.